1. Field of the Invention
The invention relates to an MR method with a sequence which is repeated several times and during which the nuclear magnetization in an examination zone is excited in the presence of a uniform, steady magnetic field 2, after which an MR signal is received from the examination zone and phase errors occur due to concomitant gradients in the time interval between the excitation of the nuclear magnetization and the reception of the MR signal. The invention also relates to a device for carrying out the method.
2. Description of the Invention
From WO 98 02 757 it is known that MR methods involve so-called concomitant gradients; concomitant gradients are to be understood to mean the gradients of the magnetic fields which extend perpendicularly to the direction of the steady magnetic field. As a consequence of Maxwell's field equations they inevitably occur together with the desired gradients of the magnetic field components extending in the direction of the steady field. Such concomitant gradients may cause image artifacts when slices are examined whose planes do not extend perpendicularly to the direction of the uniform, steady magnetic field (coronal, sagittal, or oblique slices) or when the slices are situated outside the iso-center of an MRI apparatus.
The influence exerted on the image quality by the concomitant gradients is dependent on their magnitude in relation to the steady magnetic field. In the case of strong steady magnetic fields (0.5 Tesla or more), the effect of the concomitant gradients on the image quality generally will hardly be noticeable. However, in the case of lower strengths of the steady magnetic field, for example 15 mT as occurring in the so-called Overhauser imaging methods, their negative effect on the image quality is very pronounced.
In order to compensate the phase errors caused by the concomitant gradients and affecting the image quality, the known MR apparatus utilizes five additional coils whereby correction magnetic fields are generated. These additional coils are controlled by means of pulses which must be adapted to the relevant MR pulse sequence. This solution requires a comparatively large amount of additional means.
Another solution, which is known from Proc. SMRM, London 1985, 1037-103 8, proposes to split the prephasing pulse for the read-out gradients and the phase encoding pulse into two parts wherebetween a 180.degree. RF pulse acts on the examination zone. The phase errors are then zero at the instant at which the read-out gradient is started. It is a drawback of this method that it is necessary to use a 180.degree. RF pulse in each sequence.